The generation of droplets from capillary stream break-up has been studied at least as early as Lord Rayleigh in the 1800s. More recently, the formation of metallic spheres, or balls, from the break-up of a molten metal capillary stream has been studied. Such balls are commonly used in the electronics industry for various applications, including interconnects for small electronics packages and in the manufacture of conductive pastes. Using the process of capillary stream break-up, the balls can be produced at very high rates—typically tens of thousands of droplets per second. Further, the nature of droplet formation due to capillary stream break-up results in highly uniform balls. The highly uniform size of the metal balls formed from capillary stream break-up is a significant improvement over other methods of forming conductive powders—such as spray atomization or melt spinning—which require the extra step of sieving or sorting the differently sized balls. This extra step is labor intensive, significantly increasing the time and cost of the manufacturing process; however, with such technologies, sorting or sieving is necessary to achieve tight ball diameter tolerances (on the order of five percent).
In the production of metal balls from capillary stream break-up, it is advantageous to effectively cool the balls so that they solidify before landing or bonding with each other. Effective solidification reduces or eliminates (1) irregularly shaped balls that have dented when they impinge and (2) irregularly sized balls that have bonded together because they were insufficiently cooled. Without effective solidification, removal of these defects requires that the balls be sieved or sorted.